1. Field of the Invention
The invention relates to an optical head for recording data into and reproducing data from a medium such as a phase-change type optical disc and a photo-electro-magnetic tape, a method of fabricating such an optical head, and an apparatus for fabricating such an optical head.
2. Description of the Related Art
FIG. 1 illustrates an optical pickup as an example of a conventional optical head equipped with a micro-prism. The illustrated pickup is one having been suggested in Proceedings of the 6th Sony Research Forum, "The optical features of an integrated "Laser Coupler" optical pickup in the CD system", Yoshiyuki Matsumoto et al., 1996, pp. 541-546.
A light having been emitted from a laser diode chip 101 is reflected at a first plane 104a of a micro-prism 104 by half an amount, and the thus reflected light is focused onto an optical disc 106 through a lens 105. A light having been reflected at the optical disc 106 advances on the same optical path in an opposite direction, and then, is refracted at the first plane 104a to thereby enter the micro-prism 104.
The micro-prism 104 includes a second surface 104b having a half-mirror coating in the right half thereof Hence, a light having entered the second surface 104b of the micro-prism 104 transmits through the second surface 104b in half an amount, and then, is received in a front light-receiving section 103a of a photodiode chip 103. The remaining half of a light is reflected at the second surface 104b, and enters a third surface 104c of the micro-prism 104.
The second surface 104b of the micro-prism has a non-reflective coating applied thereto in the left half. Hence, a light having been reflected at the third surface 104c of the micro-prism transmits through the second surface 104b, and then, is received at a rear light-receiving section 103b of the photodiode chip 103.
Alight emitted from the laser diode chip 101 varies in an amount due to degradation with the lapse of time and variation in temperature, even if a constant current is applied to the laser diode chip 101. Hence, a light backwardly emitted from the laser diode chip 101 is received a light-receiving section (not illustrated) formed on a sub-mount 102, and a signal detected by the light-receiving section is fed back to a current to be applied to the laser diode chip 101, to thereby keep an amount of light emitted from the laser diode chip 101 constant.
As illustrated in FIG. 2, the front light-receiving section 103a is comprised of four light-receiving sections 103aa, 103ab, 103ac and 103ad defined by three divisional lines extending in a direction from the laser diode chip 101 to the micro-prism 104 (that is, a y-axis direction illustrated in FIGS. 1 and 2) in parallel with one another. Similarly, the rear light-receiving section 103b is comprised of four light-receiving sections 103ba, 103bb, 103bc and 103bd defined by similar three divisional lines. Herein, signals to be detected in the light-receiving sections 103aa to 103bd are represented with S103aa to S103bd, respectively. A focus signal FE100 is detected in accordance with the following equation. EQU FE100=S103aa-S103ab-S103ac+S103ad-S103ba+S103bb+S103bc-S103bd
A track error signal TE100 is detected in accordance with the following equation. EQU TE100=S103aa+S103ab-S103ac-S103ad-S103ba-S103bb+S103bc+S103bd
However, the conventional optical head illustrated in FIG. 1 is accompanied with the following problems.
The first problem is that it is unavoidable for an optical head to be thick.
For instance, Japanese Unexamined Patent Publication No. 6-333290 has suggested such an optical pickup as illustrated in FIG. 3. The illustrated optical pickup is designed to include a mirror 107 to reflect a light reflected at the first surface 104a, in parallel with the photodiode chip 103, in order to make the optical pickup thinner.
However, as long as a light directing from the laser diode chip 101 to the lens 105 is to be reflected at the first surface 104a of the micro-prism 104, the pickup cannot have a thickness smaller than a sum of thicknesses of the mirror 107, the micro-prism 104, and the photodiode 103.
In addition, the optical pickup illustrated in FIG. 3 has to include a second mirror 108 at which a light reflected from the mirror 107 is reflected towards the optical disc 106, as well as the first mirror 107, resulting in an increase in the number of parts and complexity in a structure.
The second problem is that a light can be utilized in an amount only by 25% at greatest.
This is because, among a light emitted from the laser diode chip 101 to the lens 105, a light not reflected but refracted at the first surface 104a of the micro-prism 104, and a light having been reflected at the optical disc 106, and then, not refracted but reflected at the first surface 104a, are consumed in loss.
There may be employed a quarter wavelength plate in order to change polarizing directions in incoming and outgoing optical path for increasing a light utilization efficiency. However, a surface having polarization can be formed only at a surface through which mediums having almost the same indexes of refraction make contact with each other. Hence, it might occur to those skilled in the art that a pillar-shaped micro-prism having a cross-section of a right-angled isosceles triangle and having almost the same index of refraction as that of the micro-prism 104 is adhered to the first surface 104a of the micro-prism 104. However, such a structure would be accompanied newly with a problem that a focus error signal cannot be detected, because a light having been reflected at the optical disc 106 straightly advances without being refracted at the first surface 104a.
The third problem is poor productivity of the micro-prism 104.
This is because that the first surface 104a of the micro-prism has to be polished accurately at an angle of 45 degrees, and further because the second surface 104b has be coated with a half-mirror coating in one half, and with non-reflective coating in the other half.
The fourth problem is that if a light-emitting point of the laser diode chip 101 at which a light is emitted is shifted in a z-axis direction, there will be generated focus offset. This is because a height of the light-emitting point of the laser diode chip 101 is dependent on a thickness of the sub-mount 102 on which the laser diode chip 101 is mounted.
A light reflected at the optical disc 106 is designed to be converged on the third surface 104c of the micro-prism, when the optical disc 106 is located on a light-converging point of the lens 105. As illustrated in FIG. 4, if the light-emitting point of the laser diode chip 101 is shifted in a z-axis direction by a distance of "q", there is not generated an optical path difference in an incoming path, but there is generated an optical path difference D in an outgoing path, defined as the following equation, with respect to an optical path indicated with a solid line and an optical path indicated with a broken line. As a result, a light having been reflected at the optical disc 106 is not focused on the third surface 104c of the micro-prism 104. EQU D=[(2n.sup.2 -1).sup.1/2 -n.sup.2 ]q/(n.sup.2 -1)
The fifth problem is that the track error signal is likely to be mixed into the focus error signal.
This is because it is unavoidable for an optical pickup to be assembled containing focus offset therein, due to limited assembling accuracy. The focus offset is removed by means of a particular circuit, when an optical pickup is incorporated into a drive. However, after all, the optical pickup is assembled in such a manner that a beam spot formed on the front light-receiving section 103a and a beam spot formed on the rear light-receiving section 103b are different in size from each other when the optical disc 106 is located on a light-converging point of the lens 105.
The focus error signal FE100 is detected in accordance with the following equation. EQU FE100=S103aa-S103ab-S103ac+S103ad-S103ba+S103bb+S103bc-S103bd
Hence, components of the track error signal to be mixed into the focus error signal FE100 ought to be cancelled. Specifically, a component mixed into (S103aa-S103ab) ought to be cancelled with a component mixed into (S103bc-S103bd), and a component mixed into (-S103ac+S103ad) ought to be cancelled with a component mixed into (-S103ba+S103bb). However, since the optical pickup is assembled in such a manner that a beam spot formed on the front light-receiving section 103a and a beam spot formed on the rear light-receiving section 103b bare different in size from each other when the optical disc 106 is located on a light-converging point of the lens 105, the track error signal components to be mixed into the focus error signal are not cancelled, and thus, remain as they are.
Apart from the optical head illustrated in FIG. 1, Japanese Unexamined Patent Publication No. 9-73652 has suggested an optical pickup including a first beam splitter film, a second beam splitter film which allows p-polarization components to transmit therethrough by 100%, but reflects s-polarization components by a certain ratio in a direction of an optical axis of a light having transmitted through the first beam splitter film, and a quarter wavelength plate converting the thus reflected light from a linearly polarized light to a circularly polarized light, or vice versa.
However, this optical pickup is accompanied with the same problems as the problems mentioned above with respect to the optical head illustrated in FIG. 1.
Japanese Unexamined Patent Publication No. 6-302044 has suggested an optical pickup in which a light having emitted from a laser source and having transmitted through a beam splitter film and a polarized beam splitter film of an optical block is radiated onto a recording surface of a photo-electro-magnetic disc through a complicated rotatory polarization plate and an objective lens, and a reflected light is introduced into the optical block through the complicated rotatory polarization plate. The light is detected by means of a photodetector in the optical block.
However, this optical pickup is accompanied with the following problems.
In the optical pickup, the complicated rotatory polarization plate is designed to have 45 degrees of a rotatory polarization angle for detecting photo-electro-magnetic signals. However, this results in that a ratio of an amount of a light entering a first photodetector to an amount of a light entering a second photodetector is 20:3, and resultingly, a focus error signal can no longer be detected.
Even if it is given up to detect a photo-electro-magnetic signal, and the complicated rotatory polarization plate is designed to have such a rotatory polarization angle that s-polarization components are 13% and p-polarization components are 87%, in order to detect a focus error signal, a light utilization efficiency of the optical pickup is about 22% at greatest. This means that it is not worth while utilizing polarization. Utilization of polarization causes extra costs for preparing a part or parts for utilizing polarization, and makes it extremely difficult to reproduce data from an optical disc having birefringence. Hence, the optical pickup is not allowed to put into practice, considering the fact that a light utilization efficiency is low, specifically, about 22%, in spite of utilization of polarization.
Japanese Unexamined Patent Publication No. 8-7325 has suggested an optical head in which a light beam emitted from a semiconductor laser is reflected a plurality of times in first and second prisms, and then, is introduced to an objective lens through which the light beam is focused onto a recording surface of an optical disc. The reflected light is divided into two portions one of which is utilized for servo operation in focusing and tracking, and the other is utilized for detecting a photo-electro-magnetic signal.
Japanese Unexamined Patent Publication No. 8-36781 has suggested an optical head including a prism formed with a first inclined surface having a half-mirror function and a second inclined surface allowing a light to transmit therethrough. A light reflected from an optical disc is divided into a plurality of portions through a polarized light splitter and a planar prism. The thus divided portions of a light have different polarization directions. These portions of a light are focused onto a light-receiving region formed on a substrate to thereby detect data of the optical disc. At the same time, a focus error signal is detected by virtue of astigmatism which is generated when the portions of a light transmits through the planar prism.